1. Field of Invention
The present invention relates generally to the application of a colored layer to a toner image formed on a substrate and, more particularly, to an applicator for selectively applying a colored layer to a toner image.
2. Description of the Related Art
In recent years, the formation of toner images on paper and other substrates has become commonplace. Laser printers and photographic reproduction machines are common examples of devices that may be used to form toner images on a substrate. In such devices, the toner image is formed by applying toner powder to an electrostatic latent image formed on the substrate. The toner image is then fixed to the substrate by a developer which applies heat to the substrate, thereby melting the toner powder onto the substrate.
Unfortunately, common inexpensive laser printers and photographic reproduction machines are only equipped to apply black toner to the substrate. As such, those who wish to print in colors other than black must resort to the use of expensive color copiers and laser printers.
One proposed solution to this problem in the art is described in U.S. Pat. No. 4,724,026 to Nelson. The '026 patent is directed to a process for selective transfer of metallic foils to xerographic images. Referring to FIGS. 2 and 3 of the '026 patent, Nelson discloses a transfer sheet assembly consisting of a backing sheet and a transfer sheet which are secured to one another along one of the longitudinal ends of the assembly. The transfer sheet is composed of a number of layers--a carrier layer, a polymer film layer, a metallic film layer and a heat-activated adhesive layer. A substrate having a toner image formed thereon may be placed between the backing sheet and the transfer sheet with the toner image facing the heat-activated adhesive layer of the transfer sheet. Heat and pressure are then applied to all three, which causes the adhesive to bond to the toner image. The transfer sheet is then pulled away from the substrate and, as illustrated in FIG. 3 of the '026 patent, the heat activated adhesive bonds portions of the metallic film layer to the toner image.
There are a number of disadvantages associated with the process disclosed in the '026 patent. For example, an entire transfer sheet is consumed during each transfer process, thereby resulting in waste in those situations where only a small portion of the substrate includes a toner image. Also, it is impossible to position the transfer sheet so that only selected areas on the substrate are effected. Moreover, because the adhesive does not soften until heat is applied, the substrate may move relative to the transfer sheet prior to the bonding process.
Other disadvantages are related to the construction of the '026 transfer sheet. The sheet includes a number of layers of different materials (e.g., polymer film, metallic film and heat-activated adhesive layers) formed on a carrier. With the production methods described in the '026 patent, each of these layers are necessary in order to produce an opaque covering on the toner. There are, however, manufacturing costs associated with each step in any manufacturing process. As such, it would be desirable to produce an opaque toner covering which does not require as many layers of different materials.
Another specific implementation of the prior art method of applying color to a toner image formed on a substrate is illustrated in FIGS. 1 and 2 of the present application. Here, a color coating layer is formed on a strip of polyester film 10 which is stored in roll form. The strip 10 may be selectively positioned over a toner image formed on a substrate 12 through the use of adhesive tabs, or dots, 14. The application of heat and pressure to the toner image and strip 10, preferably by the fuser (or developing unit) of a laser printer or photocopier, causes the toner to soften. The softened toner binds to those portions of the color coating layer in contact with the toner. After the strip is removed, the color coating will remain on the toner image.
This method is essentially the same as that disclosed in the patent cited above with separate pressure sensitive adhesive dots being applied in a manner similar to the pressure sensitive adhesive strip used in the '026 patent. Similarly, there are a number of disadvantages associated with the prior art method illustrated in FIGS. 1 and 2. For example, the adhesive tabs, which must be individually peeled from a pad, tend to stick to the user's fingers. Also, arranging an adhesive tab on each corner of the strip is inconvenient and the tabs often come off inside the printer. Another disadvantage relates to the fact that paper (and other substrates) is not always perfectly flat when it is pulled into a laser printer from a feeding tray. Some portions may bow slightly. Because the prior art strip is only secured at the corners, the strip may bow away from the portion of the sheet which it covers. This may cause the printer to jam or cause the strip to be fully or partially peeled away from the substrate as it enters the printer. Finally, the sheet of unused adhesive tabs may become separated from the roll and lost or may be misplaced.
Finally, the prior art methods of applying color to a toner image fail to provide additional uses for the transfer sheet after some of its color has been applied to the toner.